The natural biology of the complex group of murine C-type RNA viruses is being studied using virological, genetic, and biochemical approaches. Transmission by genetic means has been delineated for a number of spontaneously produced or chemically inducible murine leukemia viruses (MuLV); these include xenotropic and ecotropic MuLV from both high and low virus expression mouse strains. In addition, the chromosomal location of several of these virus genomes has been identified. In studies of the relation of this virus group to neoplastic disease in mice, naturally occurring recombinant viruses containing determinants of both ecotropic and xenotropic MuLV have been found. These viruses are of partcular interest because of their association with the preleukemic or leukemic state, the ability of certain isolates to induce or accelerate leukemia in mice, and the opportunity they provide for analyzing virus genome structure and segment function.